Bronsted acids and pKa
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What is a bronsted acid?
Source of protons
What is a lewis acid?
Electron pair acceptor
Show dissociation of an acid HX
HX H+ + X-
What is the conjugate base of nitric acid?
NO3-
How many ionisable protons are there in nitric acid?
1
What is the conjugate base of sulfuric acid?
HSO4-
How many ionisable protons are there in sulfuric acid?
2
In the acids, which protons are ionisable?
The ones attached to the oxygen as this forms an acidic O-H bond
What is the conjugate base of phosphonic acid?
H2PO3-
How many ionisable protons are there in phosphonic acid?
2
What is the conjugate base of phosphoric acid?
H2PO4-
How many ionisable protons are there in phosphoric acid?
3
How is the strength of an inorganic acid measured?
the value of pKa
What is the equation for pKa?
pKa = -log10 (Ka)
How is Ka calculated?
Ka = [H+][X-] / HX
Lower pKa …
Stronger acid
Higher Ka …
Stronger acid
What is the position of prolytic equilibria determined by?
pKa(HX) and pKa(HY)
What happens to acidity when you go down a group?
Increases
Why do molecules become a stronger acid when they move down a group?
Orbital overlap The H-X bond energy decreases The X atom gets larger as you move down the group so the orbital overlap is worse, meaning the hydrogen atom is lost more easily
Why do compounds become a stronger acid when they move down a group?
Orbital overlap The H-X bond energy decreases The X atom gets larger as you move down the group so the orbital overlap is worse, meaning the hydrogen atom is lost more easily
What happens to acidity as you go along a period?
Increases
Why does acidity increase as you go along a period?
Electronegativity There is decreasing electronegativity of the conjugate base which means there is increasing bond energy
How does the pKa of H3C-H relate to F-H?
H3C-H = 48 F-H = 3.5